Phase Change Materials for Applications that Require Fast Switching and High Endurance

ABSTRACT

A memory device utilizing a phase change material as the storage medium, the phase change material based on antimony as the solvent in a solid solution; wherein the memory device further includes a means for heating the phase change material.

The present application is a continuation application based onpreviously filed application Ser. No. 12/047459, filed on Mar. 13, 2008.

The present invention relates to a solid solution phase change material,and more particularly, to a nonvolatile memory device employing a solidsolution phase change material as the memory element.

BACKGROUND OF THE INVENTION

Phase change materials (PCMs) undergo fast reversible phase changes inresponse to an external stimulus, such as heat. The phase change isassociated with a change in a physical property, such as electricalresistance or optical reflectivity, which can be measured to determinethe phase of the material. PCMs are typically switched between a largelyamorphous state and a largely crystalline state. The amorphous state ischaracterized by a higher electrical resistance than the crystallinestate. Switching between these two states generates a reversibledifference in electrical resistance that can be harnessed for a varietyof applications.

Materials that exhibit fast and reversible phase changes are in highdemand for many semiconductor applications. Particular applications forthis technology include memory cell devices that store binaryinformation. These devices can be classified as either a volatile memorydevice or a non-volatile memory device. A volatile memory device maylose data stored in the device when power is removed from the device. Onthe other hand, a nonvolatile memory device may retain its data evenwithout power.

PCM memory devices have the potential to compete with existing memorydevices due to their comparatively high resistivity in both theamorphous and crystalline states. High resistivities lead to a highvoltage drop and higher power deposition for a given current pulse,which in turn requires less current to switch the cell from thecrystalline state to the amorphous state and vice versa.

PCMs commonly applied in this technology include chalcogenide metalalloys composed of germanium (Ge), antimony (Sb) and Tellurium (Te), forexample, Ge₂Sb₂Te₅ (GST). Examples of metal alloys utilized as PCMsdisclosed in the pior art include: WO 2007/029938 which relates to aphase change memory device using an antimony-selenium metal alloy. Thematerial has a low melting point and high speed of crystallization. US2007/0001160 discloses a phase change memory material wherein the basematerial is an antimony-tellurium binary solution, an antimony-germaniumbinary solution, an antimony-indium binary solution or anantimony-gallium binary solution. The solutions disclosed are in theeutectic range. However, a disadvantage of conventional metal alloy PCMsis their tendency to degrade in high volume switching activity.

One promising approach for the fabrication of resistive nonvolatilememory cells is based on the use of solid solutions as an active(switching) material for nonvolatile memory cells. A memory cell of thistype has a layer of a solid solution phase change material arrangedbetween a first electrode and a second electrode.

Solid solution PCMs are known in the prior art. Solis et al (FASTCRYSTALLIZING GeSb ALLOYS FOR OPTICAL-DATA STORAGE Journal of AppliedPhysics, 1994, vol. 75, N12 June 15, pp. 7788-7794) disclose fastreversible optical storage materials in which amorphous-crystallinecycling is achieved by employing ultra-short laser pulses. Ahigh-reflectivity extended solid solution of germanium in crystallineantimony is also disclosed.

Afonso et al. (ULTRAFAST ERASABLE OPTICAL STORAGE IN Sb-RICH GeSb FILMSThe 1994 Conference on Lasers and Electro-Optics Europe, Amsterdam,Netherlands, 28 Aug.2 Sep. 1994. Publisher: IEEE Piscataway, N.J.)disclose the use of films of solid solutions of germanium and antimonyfor recording micron-sized bits for optical discs. A specific alloycomposition disclosed in the article is 13% germanium in antimony, whichis very near the eutectic. Neither of the two articles discloses the useof solid solutions for phase change memory cell devices.

None of the above-cited references, taken either alone or incombination, anticipate the present invention as disclosed and claimedherein.

SUMMARY OF THE INVENTION

A semiconductor memory device may be classified as either a volatilememory device or a non-volatile memory device. Flash memory devices,which are a type of nonvolatile memory device, are frequently used forstoring data. However, flash memory devices are not generally configuredas random access memory devices. In addition, flash memory devices aredisadvantageous in that the time required for reading or writing datafrom or to such devices may be relatively long.

The present invention relates to electrical phase-change memories. Inparticular, a system and method of operation are provided for thephase-change memory cells using a multi-pulse reset write scheme.Phase-change materials may exhibit at least two different states.Consequently, phase-change material may be used in a memory cell tostore a bit of data. The states of a phase change material may bereferred to as amorphous and crystalline states. These states may bedistinguished because the amorphous state generally exhibits higherresistivity than does the crystalline state. Generally, the amorphousstate involves a more disordered atomic structure, while the crystallinestate involves an ordered lattice.

A phase change composition, useful in fast switching and high enduranceapplications, is an embodiment of the present invention. The phasechange composition comprises a solid solution and, optionally, a dopant.The solid solution consists essentially of antimony as a solvent and asolute which is a member selected from the group consisting ofgermanium, arsenic, tellurium, strontium, sulfur, tin and mixturesthereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a phase diagram of a GeSb composition.

FIG. 2 is a representation of a memory cell containing a memory elementcomprising a solid solution phase change material.

FIG. 3 is a phase diagram of an AsSb composition.

FIG. 4 is a phase diagram of a SrSb composition.

FIG. 5 is a phase diagram of a SnSb composition.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the present invention is a phase change composition,useful in fast switching and high endurance applications. The phasechange composition comprises a solid solution and, optionally, a dopant.The solid solution consists essentially of antimony as a solvent and asolute which is a member selected from the group consisting ofgermanium, arsenic, tellurium, strontium, sulfur, tin and mixturesthereof. In a preferred embodiment, the solute is germanium. Preferably,the germanium is present in the phase change material, excluding dopant,in an amount of about 1% to about 12% based on atomic ratio. Mostpreferably, the amount of germanium present in the phase changematerial, excluding dopant, is about 8% based on atomic ratio. Thedopant, which is optionally part of the phase change composition, can bein the solid solution. However, in an alternative embodiment, the dopantneed not be part of the solid solution. When present, the dopant can bein an amount of at least about 0.01% based on weight. Examples ofdopants are: monatomic nitrogen, monatomic oxygen, silicon nitride andsilicon monoxide

The phase change composition has a crystallization temperature of about250 degrees C. to about 300 degrees C. The type of dopant, as well asamount, is selected to meet the required crystallization temperature.Time of crystallization is about 20 nanoseconds or less. Preferably, thetime of crystallization is about 5 nanoseconds to about 10 nanoseconds.In a most preferred embodiment, the time of crystallization is about 10nanoseconds.

The phase change composition has a resistivity, when in the crystallinephase, of about 0.001 ohms-cm. to about 0.1 ohms-cm. In a preferredembodiment, the phase change composition has a resistivity, in thecrystalline phase, of about 0.01 ohms-cm. In the amorphous phase, thephase change composition has a resistivity that is about 2 to about 5orders of magnitude greater than the resistivity in the crystallinephase.

Another embodiment of the present invention is a process for preparing anonvolatile memory cell device. The memory cell device contains a memoryelement comprising a phase change material having fast switchingcapability and high endurance. The process comprises the steps of:obtaining a phase change material comprising a solid solution consistingessentially of antimony as a solvent, and a solute which is a memberselected from the group consisting of germanium, arsenic, tellurium,strontium, sulfur, tin and mixtures thereof; and, optionally, a dopant;and embedding the phase change material in a substrate comprising aninsulating material. A first side of the phase change material is thencontacted with a first electrode; and a second side of the phase changematerial is contacted with a second electrode. The first and secondelectrodes are in contact with at least one word line and at least onebit line. The first and second electrodes can then co-operate to pass anelectrical current through the phase change material.

Another embodiment of the present invention is a phase change memorycell device comprising a phase change material comprising a solidsolution and, optionally, a dopant. The solid solution comprises asolvent and a solute. The solvent is antimony and the solute is a memberselected from the group consisting of germanium, arsenic, tellurium,strontium, sulfur, tin and mixtures thereof.

Another embodiment of the invention is a device that includes a phasechange material that includes the elements Ge and Sb, existing as asolid solution, as well as, optionally, at least one dopant. The dopantcan be a nitride compound. The dopant constitutes at least 1 atomicpercent of the phase change material. The dopant and concentrations ofthe dopant and the elements are selected so that the phase changematerial has a crystallization temperature of at least 250 degrees C.and a resistivity, in the crystalline phase, of at least 0.001 ohm-cm.In an exemplary embodiment, said solid solution and dopant, as well asthe concentrations of the dopant and said solid solution, are selectedso that the phase change composition has a crystallization temperatureof at least 270 degrees C. and a resistivity, in the crystalline phase,of at least 0.01 ohm-cm.

The memory device further includes a component that includes at leastone of the following: a) electrodes for passing current through thephase change material, thereby heating the phase change material, b) ascanning probe microscopy-based element for applying heat to the phasechange material, and c) a laser having output used to heat the phasechange material. This component enables at least one of the following:reading data from the phase change material and writing data into thephase change material. In one preferred embodiment, electrodes aredisposed on different sides of the phase change composition forconducting electrical current through the phase change material. Asubstrate underlies the phase change material. A capping layer coversthe phase change composition.

In theory, the present invention is based on the concept that antimonyalone is a very fast phase change material. However, due to the natureof the element, a PCM with pure antimony is desirable but not practical.A minor amount of another elemental material therefore is needed inorder for the antimony to function in a device employing a phase changecomposition. The other material can be a chalcogenide, but it does nothave to be a chalcogenide. The minor amount of added material must forma homogeneous phase with the antimony, hence a solid solution. Dopantthat can be present in the phase change composition cannot be present inan amount that will destroy the solid solution.

A solid solution relates to a composition where there is present on theatomic level a solid-state solution of at least one solute in a solvent.The solid-state solution is a mixture that is a solution rather than acompound because the crystal structure of the solvent remains unchangedupon addition of the one or more solutes. The mixture remains in asingle homogeneous phase.

There are two possible methods of incorporation of the solute into thesolvent. A solvent particle in the lattice can be replaced by solute.This is known as the substitutional method. The second method is theinterstitional method, whereby a solvent particle fits into the spacebetween solvent particles in the lattice. In both cases, the crystallattice of the solvent is distorted, thus disrupting the physical andelectrical homogeneity of the solvent material.

In order to form solid solutions, the solute and solvent have certainfeatures such as: similar atomic radii (15% or less difference), samecrystal structure, similar electronegativities, and similar valency.Even small amounts of solute can affect the electrical and physicalproperties of the solvent.

FIG. 1 relates to an equilibrium phase diagram of a germanium-antimonycomposition wherein the solid solution is obtained when the amount ofgermanium is present in an amount from trace to about 12% based onatomic ratio. The antimony is the solvent in the solid solution. Thesolid lines in the diagram relate to the phase diagram, and the dashedlines relate to the tentative metastable equilibria involving germaniumand antimony. One embodiment of the present invention is a method ofpreparing a phase change memory cell device wherein the storage mediumcomprises a phase change composition that is a solid solution ofgermanium in antimony. In a most preferred embodiment, the solidsolution consists of 8% germanium-antimony, wherein the percentagerefers to the amount of germanium present in the solid solution based onatomic ratio.

FIG. 2 is a representative drawing of a phase change memory cell device.A top electrode 11 is positioned next to one side of a phase changememory element 13. A bottom electrode 12 is positioned next to otherside of the phase change memory element 13. The phase change memoryelement 13 comprises a solid solution of antimony as the solvent, and asolute which is a member selected from the group consisting ofgermanium, arsenic, tellurium, strontium, sulfur, tin and mixturesthereof. The phase change memory element 13 also contains at least onedopant material. The dopant can be part of the solid solution. Thedopant can, alternatively, be separate from the actual solid solution.The memory element 13 is embedded in an insulation layer 14.

FIG. 3 relates to an equilibrium phase diagram of an arsenic-antimonycomposition wherein the solid solution is obtained in all concentrationsof arsenic and antimony. The area represented by (Sb,As) is the solidsolution phase of the binary alloy. The area below the dotted linehaving question marks above the line represents concentrations where thesolid solution phase is questionable. The present invention includes allpossible concentrations of arsenic, as long as the antimony in thebinary alloy is the solvent in the solid solution.

FIG. 4 relates to an equilibrium phase diagram of a strontium-antimonycomposition wherein the area designated as (Sb) is the solid solutionphase of the binary alloy. Thus, the amount of strontium in the solidsolution is from trace to about 2% based on atomic ratio.

FIG. 5 relates to an equilibrium phase diagram of a tin-antimonycomposition wherein the area designated as (Sb) is the solid solutionphase of the binary alloy. Thus, the amount of tin in the solid solutionis from trace to about 12% based on atomic ratio.

A phase change memory element is locally heated to a temperature higherthan the melting point and then rapidly cooled to form amorphous marks.On the other hand, the memory element is heated at a temperature ofapproximately at most the melting point and at least the crystallizationtemperature, and slowly cooled so that the memory element is kept at atemperature of at least the crystallization temperature for a certainretention time to carry out recrystallization. Namely, a reversiblechange between the stable crystalline phase and the amorphous phase isutilized, and the information is recorded or retrieved by detecting thedifference in physical parameters such as refractive index, electricresistance, volume and change in density, between the crystalline stateand the amorphous state.

Phase change in the phase-change materials may be induced reversibly. Inthis way, the memory may change from the amorphous to the crystallinestate, and vise versa, in response to temperature changes. Thetemperature changes to the phase-change material may be effectuated in avariety of ways. For example, a laser can be directed to thephase-change material, current or voltage may be driven through thephase change material, or current or voltage can be fed through aresistive heater adjacent the phase change material. With any of thesemethods, controllably heating the phase-change material causescontrollable phase change with the phase-change material.

Phase-change compositions of the present invention comprise solidsolutions. Preferably, the solid solutions contain antimony. In a mostpreferred embodiment of the present invention, the antimony in the solidsolution is the solvent. That is, the solid solution is antimony-based.

When a storage medium comprising an antimony-based solid solution isemployed in a memory cell device, switching times and the switchingenergies required for the transitions between the amorphous and thecrystalline states are substantially reduced below those attainable withprior art electrically erasable phase change memories. One embodiment ofthe invention comprises an integrated circuit implementation of thememory in a high bit density configuration in which manufacturing costsare correspondingly reduced and performance parameters are furtherimproved.

Generally, not all of the phase change material in a memory cell deviceundergoes phase change. The portion of the layer of the phase-changeablematerial where the phase change occurs may be referred to as the“programming region.” When the programming region has an amorphousstate, the state of a PRAM (phase change random access memory) device isreferred to as the “reset state.” On the other hand, when theprogramming region has a crystalline state, the state of the PRAM deviceis referred to as the “set state.” The resistance of the portion of theprogramming region making contact with the lower electrode may berelatively high in the reset state. On the other hand, the resistance ofthe portion of the programming region making contact with the lowerelectrode is relatively low in the set state.

Another embodiment of the present invention relates to a phase changememory device having a phase-changeable material layer pattern. Thisembodiment of the invention provides for a phase change memory deviceincluding a phase-changeable material layer pattern, wherein the phasechange material comprises an antimony-based solid solution. Thus, inmemory devices according to this embodiment of the invention, someelectrical characteristics of the phase-changeable memory device may beimproved. Additionally, cells of a phase change memory device formed inaccordance with this embodiment of the invention may be highlyintegrated.

Another embodiment of the present invention relates to a method ofmanufacturing a phase change memory device comprising: forming aninsulating interlayer on a silicon substrate; forming a silicon nitridelayer on the insulating interlayer; forming an opening in the siliconnitride layer and the insulating interlayer; forming a lower electrodein the opening, the lower electrode making contact with a contact regionof the substrate through the silicon nitride layer and the insulatinginterlayer; forming a phase-changeable material layer, wherein the phasechange material comprises an antimony-based solid solution, on the lowerelectrode; and forming an upper electrode on the phase-changeablematerial layer.

Another embodiment of the present invention relates to a phase changememory device comprising: a substrate of silicon having a contact regiontherein; an insulating interlayer on the substrate; a silicon nitridelayer on the insulating interlayer; a lower electrode extending throughthe silicon nitride layer and the insulating interlayer and inelectrical contact with the contact region of the substrate; aphase-changeable material layer pattern, wherein the phase-changeablematerial is an antimony-based solid solution, on the lower electrode andthe silicon nitride layer; and an upper electrode on thephase-changeable material layer pattern.

Various phase change materials based on solid solutions are disclosedherein that may be advantageously used as the memory element in a deviceused for information storage and/or retrieval. In one embodiment, thedevice includes a solid state memory cell array, in which each cell hasphase change material (PCM) situated between two electrodes throughwhich current is passed. Electrical current is used to selectivelychange the phase of the PCM in the memory elements, thereby recordingdata in them. Current is also used for determining the phase of the PCM,thereby allowing data to be read from the elements. In anotherembodiment, a laser is used to record data in, or read data from, alayer of phase change material. In yet another embodiment, a scanningprobe microscopy-based element is used to write data into, or read datafrom, a layer of PCM.

The memory cells themselves can be rather complicated. In oneembodiment, each cell includes a MOSFET (metal oxide semiconductor fieldeffective transistor) and a phase change storage element that includesPCM. Each phase change storage element, comprising a solid solutionbased on antimony, is in electrical communication with a particularwordline and a particular bitline. The MOSFET is connected to the phasechange storage element via a conductive line. The gate of the MOSFET isconnected to a wordline. The phase change memory element is alsoconnected to a bitline via another conductive line. In an alternativeembodiment, a simplified memory cell can be employed in which the cellconsists only of a phase change memory element tied directly to arespective wordline and a respective bitline.

In still other embodiments, the memory cell may include a bipolarjunction transistor (BJT) and a phase change memory element, wherein thephase change storage material comprises a solid solution whereinantimony is the solvent. In yet another embodiment, a diode and a phasechange memory element are included in the memory cell. Anotherembodiment of the present invention includes a memory cell that containsa tunneling diode and a phase change memory element.

The electrodes in the memory device can be formed from TiN, TiW, W, oranother electrically conducting material. The electrodes are inelectrical communication with the wordline and the bitline. By passingcurrent through the phase change material, it can be programmed into adesired state (e.g., amorphous or crystalline) or have its state readout. Phase change memory elements can also include additional layers,such as heater layers and buffer layers.

The memory element can be formed by various methods known to one ofordinary skill in the art. One such method is reactive sputtering ofvarious elemental targets in a mixture of nitrogen and a noble gas suchas argon. Another method is by employing a compound phase changematerial target such as Ge(8%)Sb(92%), and co-sputtering the dopant in,for example, a gas mixture of nitrogen and a noble gas. Alternatively,the phase change memory element can be formed by sputtering from acompound target that contains all the required elements. The dopantconcentration can be adjusted by varying the sputter conditions, such asthe gas flow rate, the sputter power of the respective guns, and thecompositions of the sputter targets themselves.

Other deposition methods can include: thermal evaporation, laserablation, chemical vapor deposition, and spin-coating deposition. Thememory element can advantageously have a dopant concentration that isnot uniform throughout the phase change material.

Information is written into the phase change material of the recordinglayer by passing an electric current through the phase change material.If the phase change material of the memory device is initially amorphousand then heated to above the crystallization temperature for a longenough time, a phase transformation from the amorphous state to thecrystalline state is induced. On the other hand, if crystalline phasechange material is heated with an even higher current and then quicklycooled (melt-quenched), the phase change material is transformed back tothe amorphous state.

Information is read by a small electrical current that passes throughthe phase change material. This current is too small to induce a phasetransformation but still large enough to detect the resistivity of thephase change material, thereby determining whether the phase changematerial is in the amorphous or the crystalline state.

A phase change memory cell device is hereby disclosed. The devicecomprises a phase change material comprising a solid solution consistingessentially of antimony as a solvent, and a solute which is a memberselected from the group consisting of germanium, arsenic, tellurium,strontium, sulfur, tin and mixtures thereof. The solid solution canoptionally contain a dopant, as long as the dopant does not destroy thecrystal structure of the antimony solvent. Further, the solid solutionis a mixture that is a solution rather than a compound. The devicefurther comprises a component for applying heat to the phase changematerial. The component is a member selected from the group consistingof electrodes, a scanning probe microscopy-based element, a laser andcombinations thereof. In a preferred embodiment, the component is a topelectrode and a back electrode. The crystal structure of the solventremains unchanged upon addition of the one or more solutes, and themixture remains in a single homogeneous phase.

EXAMPLE

A phase change composition is prepared by forming a thin film bydeposition of said film on silicon wafer substrate through theco-sputtering of germanium and antimony in the presence of a gaseousmixture of argon and nitrogen. The silicon substrate is previouslycoated with a layer of silicon oxide at a thickness of about onemicrometer. The composition of the thin film is determined by means suchas Rutherford Back Scattering Analysis. The film contains a solidsolution of antimony and germanium. The content of the germanium isabout 8% based on atomic ratio. Resistivity as a function of temperatureis then measured by forming two contacts on the sample, the contactshaving well-defined geometry. The resistivity between these contacts isthen measured while heating the sample in a nitrogen atmosphere toprevent oxidation. The resistivity is about 0.01 ohms/cm.

While the invention has been described by specific examples andembodiments, there is no intent to limit the inventive concept except asset forth in the following claim.

1. A phase change memory cell device comprising: a phase change materialcomprising a solid solution consisting essentially of antimony as asolvent, and a solute which is a member selected from the groupconsisting of germanium, arsenic, tellurium, strontium, sulfur, tin andmixtures thereof and, optionally, a dopant; wherein the solid solutionpresents a crystalline phase and an amorphous phase, and wherein thecrystalline phase has the crystal structure of the solvent; and acomponent for applying heat to the phase change material, wherein thecomponent is a member selected from the group consisting of electrodes,a scanning probe microscopy-based element, a laser and combinationsthereof.